How far away does a 400 watt hps light have to be from the top of the plants?

put your hand between the light and your plants. if it is too hot for your hand, it is too hot for the plant. adjust accordingly. i keep my 600 watt 9 inches away. but that is when the plant is at full maturity.

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well I dont have the light up with the plants yet so I cant do the hand test. Im building a new grow room but I have to dig out the bottom a little and I need to know how far i need to dig be for I put plastic down.

you probably should give yourself 18 - 24 inches of space. make sure you can adjust your light up and down. young plants can take a light about 2' away.

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ya start with the light 2 feet away and then move it closer to the canopy with your hand between the light and canopy and stop lowering the light when it gets hot to the back of your hand

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I used a 400w HPS for the flowering stage only and i kept it between 10-14 inches away for the first 4 weeks. ONce i noticed the branches getting stronger i went no closer than 6-8 inches. Hope it helps i yielded pretty well and did not burn the plants at all.

Also remember that if you get an air cooled hood for your light you can keep the light a lot closer to your plants. The closer you can safely keep your light to your plants the more lumens you are using (making for more efficient growing). I have a 400W HPS WITHOUT an air cooled hood and I can manage to keep it about 18" away from the tops without any damage.

I have an air-cooled reflector with a 400 HPS and keep it VERY close. As a matter of fact, I had a growth spurt on a couple of the ladies and when I checked them they were touching the glass. No heat stress. I can actually touch the glass and it's just warm.

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i have a floralux 400 watt hps air cooled..and the colas are 2in from the glass...huge colas!

without it being air cooled i would agree with 12-24 inch depending on the temp in ur grow room....and the heat radiated down from the bulb.

Hey man what I did with my hps is when I first moved from flourescent to my hps, I started the light four foot above the plant so as not to send it into light shock after time under a cfl. Then over the next several days I moved it down a foot at a time, until I could finally get it as close to the plant as I safely could without scorching the leaves. I am using a Gavita bulb with a built in reflector that puts off a little less heat and I am able to keep it about four to six inches off of the top of the plant without burning. As the plant grows you do need to adjust with the plant, but you don't want to raise it too high each move cause then you cause stretching wich is putting all over your growth into getting taller instead of more vegetation! My particular plant can reach anywhere between 5-7 ft indoors, My closet with an 8 ft ceiling is about perfect for the latter, with not much room to spare. I hope this shed some light on the subject!

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